High magnification projection system



35o-w SW ROSS REFERENCE n SEARCH RGD@ June 6, 1950 F. E. Ross 2,510,419

HIGH ucmncATIoN PROJECTION sYs'rEu Filed oct. 11, 194e FRANK E. R055.

T INVENToR.

By MM b A T TORNEK Patented June 6, 1950 HIGH MAGNIFICATION PROJECTION SYSTEM Frenk E. non, Almen, com minor to mmm M. cuter, Los mles, cam.

Application omhei- 11, ma, semi No. saale (c1. ss-zi) 4Claims.

The invention relates to a re-projector and more particularly to means for projecting a comparatively large image on a screen with a relatively short throw or in a relatively short distance.

More particularly the invention relates to a re-projector wherein a picture from a slide or from a motion picture illm is projected with a comparatively small manlcation to a focal p1ane,`with a second projection lens having that focal plane at its back focus for re-projecting the picture with large magnification.

If a translucent screen is employed at the above mentioned focal plane. that isjat the ,front focus of the first projection lens and at the back focus of the second projection lens, such screen disperses and wastes a substantial amount of light resulting in a decrease in the brightness of the nal picture. particularly at the margin.

An object of the invention is to increase the brightness of the picture. and this is accomplished by employing a positive lens or lenses at the above mentioned focal plane. This lens may be called a field lens and. as it is at a focal plane, it does not modify the magnification of the system but it does increase the brightness of the projected picture particularly at the edge `of the ilcld, by diverting into the second projection lens those image rays which would otherwise be lost due to dispersion when a screen is used.

Another object of the invention is to facilitate locating the above mentioned focal plane for proper location of the held lens. This is accomplished by temporarily employing a translucent screen to locate the focal plane there by rela'- tively moving the screen and the two projection lenses. whereupon the screen is removed and the ileld lens or lenses substituted therefor.

For further details of the invention reference may be made to the drawings wherein Fig. 1 is a side view in elevation partly in section of a conventional projector, with a ileld lens and reprojector carrying out the present invention.

Fig. 2 is an end view in elevation, partly in section on line 2-2 of Fig. 1 looking in the direction of the arrows.

Fig. 3 is an enlarged partial sectional view on line I-I of Fig. 1 looking in the direction of the arrows.

Pig. 4 is a schematic diagram of the optical arrangement in Fig. 1.

Mg. 5 is an enlarged fragmentary sectional view of the slide mount for the ground glass.

Fig. 8 is a sectional view corresponding to Fig. t with a modified type of field lens.

Referring in detail to the drawings, the projector l represents a conventional projector for slides or motion picture illm. If slides are employed. they may be inverted to obtain an erect image projected by objective lens 44 on the screen VIl indicated in Fig. 4. It the usual motion picture projector is employed for the projector indicated at i, the image may be erected by employing an erector prism 10, for example of the Brashear-Hastings type. although other types of image inverters imay be employed. The inverter Il has a suitable mount not shown and it is arranged close to the objective lens 2 of the projector I. The objective 2 is adjustable for focusing purposes and, for example, the lens 2 may have a lens barrel having a spiral groove l cooperating with a ball not shown in the lens support I, although other types of adjustment may be employed. n

As shown in Fig. 4 the projector l has a lamp l. preferably of high intensity, reflector t, condenser lens 1 and the transparent picture medium l which may be motion picture film in the case of a motion picture projector, or it may be one frame of a nlm. or a glass slide in the case of a. still projector. V

If the picture projected by projector I were focused directly to a screen il, see Fig. 4. it would be necessary to have projector I at a comparatively great distance from projection screen l0 which can be either opaque or translucent. For example, if the picture medium l is 35 mm.rfilm, to produce an image 30 feet in width on screen Il. screen Il must be in the neighborhood of 17 0 feet from the projector. This is wasteful o f space, particularly. in a motion picture studio where pictures are projected on to translucent screens for process shots and if such long projection throws were not required, the stage space could be used for stage settings or for other purposes. According to the present inventioma large saving is made in this projection throw with improved light efficiency as follows. If a 35 mm. movie film is to be enlarged on screen i0 with a relatively short throw, and assuming that the focal length of projector i is 5 inches, its focus is adjusted. for example, to project an aerial image say 40 inches in front of projector i. The inverter 'N is used or not, as above described. 'I'he aerial image produced by the objective 2 is picked up by and is arranged at the back focus of the projection lens 44. For example. the back focal length of the projection lens 44 may be 12 inches. Assuming that the horizontal width of the negative image on the picture I is 22 mm. or

0.87 inch, then the width of the aerial image will be eight times this or about 'I inches. 1f the image projected on screen ii is to be the same as in the other case. namely 30 feet. then the screen Il will be only 51 feet from the projection lens M instead of 1'10 feet as in prior practice. This is accomplished with increased intensity of the picture on screen il as follows. The picture t appears with initial magnification as the aerial image at the locus of 2|, and the front projection lens 44 gives further magnification at screen Il. A positive lens, here called a field lens, as indicated at 2i in Fig. 4 is arranged at or near the aerial image produced by the lens 2 and at thebackfocusoflens whichisinafocal plane common to both the projection lenses 2 and". Asshowninliig.6.theneldlens 2i may be a single double convex lens. As shown in l'lg. l, the field lens may be two condemer lenses indicated at 8|, l2. each being piano-convex, with their plane surfaces face to faceand nearly or not quite in contact. The aerial image from objective 2 is positioned midway between the lenses 8| and 82 or at the median plane of the single lens indicated at 2| in Fig. 6.

The focal plane common to projection lenses 2 and 44 is determined by casting the aerial image of the picture I onto screen 80, see Fig. 5, which may be of ground glass or other translucent material. Screen 90 is centrally mounted in a slide 2l which slides laterally into a holder l2 suitably mounted at the rear sl of a rectangular frame Il. The iront of frame Il is connected by a bellows II to the lens board il. 'Ihe lenses Il, l2 are centrally mounted in a similar slide It and the single field lens 2l is centrally mounted in a similar slide l1.

The properlooation of the eld lens is conveniently determined as follows:

When screen tl is in place in the holder I2. the positionofthisscreencanbeadjustedasitis carried by a carriage 22 having inwardly extending feet 23 and 24 which slide in grooves 25 and 2|. respectively, in a base slide 21. The slide 21 has a sliding dovetail connection 2t, 29 with the base Il. By operating hand wheel 3l which has a screw I2 and a nut connection not shown with slide 21. slide 21 can be moved back and forth to vary the position of the re-projector 2l as a whole. The position of screen 90 can be separately adjusted by operating knob 33 on carriage 22. Knob 33 has a pinion 24 which engages with a rack on the slide 21. 0n the side opposite to that shown in Fig. l, carriage 22 has another knob like u to operate a pinion like 3l for a parallel rack 2l shown in Fig. 2. The knob 3l, and its companion at the other side of mount 22, do not appear in Fig. 2 as this is a front view. The two knobs Il and Il shown in Fig. 2 are in the mount of carriage 43 for the objective lens 44. Knob Il operates a pinion I! which operates on the Yrack 3l and knob li operates a pinion 4I on the rack Il. By operating the knobs Il and Il, the objective lens 4I can be individually moved back and forth. Projector i can be moved back and forth also, hence any desired adjustment can be obtained. The opposite sides of lens board il have bolts like Il which operate in slots like l2 in the mount 43. Each bolt like Il has a nut like Il to clamp the board il in its adjusted elevated position to frame the picture on screen In. The companion nut for Il is shown at 54 in Fig. 2.

After lenses 2 and 4I and screen 90 have been relatively adjusted to focus the aerial image of 4 and of the desired size onscreen Il, the screen Il is removed, and the held lens 2l or li, I2 is substituted therefor in the holder l2.

The field lens 2l or Il and t2 may. for example, be about 12 inches in diameter and the sise of the aerial 'image projected thereon by projection lens 2 may be about 7 inches by 9 inches.

The increase in illumination of the picture projected on screen Il, when using a ,neld lens instead of a translucent screen for the aerial image is shown by the following test. A 35 mm. still nim was used at l in Fig. 4, with a comparatively low intensity lamp. i. e., i60 watt lamp, at I. When using a ground glass at 2 I, the picture projected on screen Il was so low in intensity that a reading could not be obtained on a photometer. Thegroundgiasswasremovedandafieldlens in the form of two 14 inch diameter condenser lenses was substituted therefor as indicated at 8l. I2 in Fig. 5 and the light intensity of the picture on screen Il was then two and one-half foot candles, the other elements in the system being thesame. Inthiscasethesizeoftheaerialimage was about 8 inches by i0 inches and the picture projected on screen Il was about l2 feet by 14 feet with a projection throw, that is distance from projector I to screen Il, of about l6.i'eet.

Various other modiiications may be made in the invention without departing from the spirit of the following claims. Either. or both of the lenses 2 and Il may be wide angle lenses. Also, screen lli may be translucent or opaque, e. g., for process shots or for front projection.

I claim:

i. Optical apparatus comprising a picture projector, and means for obtaining for said projector a shorter projection throw than that required for direct projection from said projector to a screen, said means comprising a projection lens for said projector for projecting an aerial image of the picture with magnification, a second projection lens for projecting said image on a screen displaying that image with further magnification wherein the picture area at said screen is at least several hundred times greater than the area of said aerial image, and a positive field lens between said projection lenses and arranged substantially at the locus of said aerial image for increasing the brightness of the picture on said screen, a slide, rack means on said slide, an adjustable suD- port having a pinion cooperating with said rack means for adjusting said field lens as a unit. an independent adjustable support having a pinion engageable with said rack means for independently adjusting said second projection lens, and means for operating said slide.

2. Optical apparatus comprising a picture projector, and means for obtaining for said projector a shorter projection throw than that required for direct projection from said projector to a screen, said means comprising a projection lens for said projector for projecting an aerial image of the picture with magnification, a second projection lens for projecting said image on a screen displaying that image with further magnincation wherein the picture area at said screen is at least several hundred times greater than the area of said aerial image, a positive field lens between said projection lenses and arranged substantially at the locus of said aerial image for increasing the brightness of the picture on said screen, said field lens comprising positive lens elements closely arranged back to back substantially in contact,

and means for adjusting said ileld lens as a unit picture l on screen Il with the picture in focus 7s relatively to said projection lenses:

3. Gptical apparatus comprising a picture projector, and means ior obtaining forlsaid projector a shorter projection throw than that required for direct projection from said projector to a screen. said means comprising a projection lens for` s'aid projector for projecting an aerial-image of the picture with magniilcation, a second projection lens tor projecting said image on a screen displaying that image with further magniilcation wherein the picture area at said screen is at least several hundred times greater than the area of said aerial image. a positive ileldlens between said projection lenses and arranged substantially at the locus of said aerial image for increasing the brightness of the picture on said screen, said deld lens comprising positive lens elements closely arranged backto back substantially in contact. and an erection prism between the projection lens oi said projector and said field Vlens.

4. Optical apparatus comprising' a picture projector, and means for obtaining for said projector a shorter projection throwv than that required forv direct projection from said projector to a screen, saidmeans comprising a projection lens oi' the picture with magniilcatioma second prothan the area o! said aerial image, and a positive field lens between said projection lenses and arranged substantially at the locus of said aerial image for increasing the brightness of the picture on said screen, said field lens comprising twov positive plano-convex lens elements closely arranged with the plano-surfaces in back to back relationship and substantially in contact. l

v FRANK E. ROSS.

REFERENCES CITED The following references are of record in the ille of this patent:

UNrrsD s'rA'rEs PATENTS Number Name Date 994,276 Reilly June 6, 1911 1,199,943 Taylor Oct. 3, 1918 1,687,030 Mitchell Oct. 9, 1928 1,783,998 Chretien Dec. 9, 1930 1,931,228 Kitroser Oct. 17, 1933 2,391,430 Macek Dec. 25, 1945 

